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A/C goes flakey with "low pressure" cut-off

For the last three or four months, the A/C in my 1993 90 has started to go
a little flaky... Any insight from the resident A/C gurus would be much

The problem is that the compressor cuts in and out sporadically. The A/C
control head shows no DTCs, but (when the compressor cuts out) indicates that
the "low pressure" switch is responsible. This typically only happens
under sustained usage on the highway, with ambient temps. > ~80F. Around
town, the compressor runs continuously regardless of the cooling load.
When conditions are cooler, it will also run continuously in sustained highway
use. When the compressor is running, the systems cools as strongly as ever.

I would have suspected (another) leak in the system and a genuine low-pressure
situation, except that it's been in this state for several months now. If
there was a leak, I would have thought that enough refrigerant would 
have been lost by now that the compressor would never even engage; or at least
that cooling capacity would be severely impaired.

Also, it seems like the conditions under which it cuts out would correspond
with the highest system pressures, not the lowest. (At least that's my
understanding of A/C systems: heavy cooling load = higher pressures.)

I found what I believe to be the connectors for the high- and low-pressure
switches: a red connector and a yellow connector, each attached to
devices screwed into the filter/dryer. I cleaned the contacts in each 
connector, which showed signs of arcing and burning. This doesn't seem
to have improved matters, however.

Any ideas?

1993 90CS 70k miles